Hess's Law
Hess's law of constant heat summation, is a relationship in physical chemistry named after Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840. The law states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps. Tossup Questions # This law can be used to calculate the acidity of the hydrohelium ion. This law can be used to find lattice energy in a process involving the formation of an ionic solid from a metal and a non-metal, called the Born-Haber Cycle. Since the quantity it concerns is a state function, this law is also equal to the differences in the * enthalpies of the products and the reactants. Also called the "law of constant heat summation," this law states that the enthalpy of a reaction is equal to the sum of the enthalpies of the individual steps of the reaction. For 10 points, name this law in thermochemistry named for a Swiss-born Russian chemist. # This law explains why a catalyst does not alter the equilibrium position of a reaction. Given data such as electron affinity, heat of vaporization, and heat of ionization, this law is used in Born-Haber cycles to calculate the lattice energy of an ionic solid. This law can also be used to calculate heats of formation. It states that enthalpy is a state function, which is why it is known as the law of constant heat summation. For 10 points, name this thermodynamic law which states that the enthalpy change of a reaction equals the enthalpy of the products minus that of the reactants, named for a Swiss-born Russian chemist. # This statement is invoked in the Born-Haber (BORN HAW-ber) cycle. This statement follows from the fact that the quantity it concerns is a state function, and therefore its value is independent of the system's trajectory. This statement is often framed in terms of the heats of formation of reactants and products. For 10 points, name this statement of thermodynamics that suggests that a reaction's enthalpy change is equal to the sum of the enthalpy changes of each step. # This law is used to determine entropy and free energy in the Bordwell thermodynamic cycle. As a consequence of this law, one can calculate lattice energy in the Born-Haber cycle. This law works because the quantity it concerns is the sum of internal energy and quantity pressure times volume and is thus a state function. As a result, this law allows for addition of the changes in that quantity for a series of stepwise reactions to determine the overall change in that quantity. For 10 points, name this law which states that adding the change in enthalpy of each step of a reaction yields the total change in enthalpy for the reaction. # A 2008 paper saw improved accuracy on the Shafizadeh model when this law was applied to Sri Lankan waste disposal in determining biomass yield, and its formulator notably showed that the oxidation of sugars leads to saccharic acids. This law holds because the quantity it concerns equals the sum of internal energy and pressure times volume, two state functions, meaning that quantity is path-independent, and when applied, this law necessitates that reversal of reactions leads to a change in sign. For 10 points, identify this law, which holds that the enthalpy change of a reaction may be calculated from the enthalpies of a series of intermediate reactions. # The Bordwell cycle extends this principle to a related quantity. The quantity that this rule concerns is the sum of internal energy and the product of pressure and volume, though that quantity, governed by this law, is better known for a change in value that is equal to TdS plus Vdp. This law may be used to calculate a compound's lattice energy via the Born-Haber cycle. For 10 points, identify this prediction, which relies on the fact that a certain thermodynamic quantity is a state function, and suggests that the change in enthalpy is independent of the path from reactants to products.